Chloroform in the Hydrologic System—Sources, Transport, Fate, Occurrence, and Effects on
Human Health and Aquatic Organisms

By Tamara Ivahnenko and Jack E. Barbash

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5137

Abstract

Chloroform is one of the volatile organic compounds (VOCs) detected most frequently in both ground and surface water. Because it is also one of the four trihalomethanes (THMs)
produced in the highest concentrations during the chlorination of drinking water and wastewater, the frequent detection of this compound in ground and surface water of the
United States is presumed to be caused primarily by the input of chlorinated water to the hydrologic system. Although anthropogenic sources of the compound are substantial, they
are currently estimated to constitute only 10 percent of the total global input to the hydrologic system. Natural sources of the compound include volcanic gases, biomass burning,
marine algae, and soil microorganisms. Under most conditions (except in the presence of unusually high bromide concentrations), chloroform is the THM produced in the highest
concentrations during chlorination. Furthermore, in most cases where more than one THM is produced from chlorination, the relative concentrations among the different compounds
usually decrease with increasing bromination (chloroform > dichlorobromomethane > chlorodibromomethane > bromoform). This phenomenon is presumed to be responsible for the common
observation that when more than one THM is detected during investigations of the occurrence of these compounds in the hydrologic system, this same trend is typically observed among
their relative concentrations or, for a uniform reporting limit, their relative frequencies of detection. This pattern could provide a valuable means for distinguishing between
chlorinated water and other potential sources of chloroform in the environment.

Chloroform has been widely detected in national, regional, and local studies of VOCs in ground, surface, source, and drinking waters. Total THM (TTHM) concentrations of the compound,
however, were typically less than the Maximum Contaminant Level (MCL) of 80 µg/L (micrograms per liter) established by the U.S. Environmental Protection Agency (USEPA) for TTHMs. In the
studies that compared land-use settings, frequencies of detection of chloroform were higher beneath urban and residential areas than beneath agricultural or undeveloped areas. Because
chloroform is a suspected human carcinogen, its presence in drinking water is a potential human health concern. Liver damage, however, is known to occur at chloroform exposures lower
than those required to cause cancer, an observation that has been considered by the USEPA as the basis for setting a new, non-zero Maximum Contaminant Level Goal of 70 µg/L for the
compound. As part of its National Water-Quality Assessment Program, the U.S. Geological Survey has been assembling and analyzing data on the occurrence of VOCs (including chloroform)
in ground and surface water on a national scale from studies conducted between 1991 and the present. This report presents a summary of current (2004) information on the uses, sources,
formation, transport, fate, and occurrence of chloroform, as well as its effects on human health and aquatic organisms.

Contents

Abstract

Introduction

Purpose and Scope

Acknowledgments

Production and Use of Chloroform

Sources of Chloroform

Direct Releases

Formation of Chloroform and Other Trihalomethanes as Disinfection By-Products

Chlorinated Water as a Source of THMs in the Hydrologic System

Formation of Chloroform from the Dehalogenation of Tetrachloromethane

Natural Sources

Anthropogenic Fluxes in Relation to Global Emissions

Physical and Chemical Properties Controlling the Transport and Fate of Chloroform

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Send questions or comments about this report to the author, T. Ivahnenko (605) 394-3238

For more information about USGS National Water-Quality Assessment
Program, visit the NAWQA
Program home page or more information about the USGS National Water-Quality
Assessment Program Volatile Organic Compound National Syntheses, visit the VOC
National Synthesis home page.